The invention relates to riser pipes of the type found in wells used for monitoring the quality of ground water. Such wells are provided in many areas such as around landfills and other places where it is environmentally desirable to constantly check for possible contamination of the water table. Periodically, containers are lowered down a well to permit sampling of the quality of the water. In many areas, the wells are quite accessible so it would appear desirable to cap them except when samples are being taken. Capping would prevent vandals from dropping foreign material into the well which could destroy its usefulness. However, it would also be desirable that the riser pipe be vented to the atmosphere at all times.